Hot melt adhesive compositions are typically thermoplastic materials which are solid at room temperature. When heated to liquid or molten form, the hot melt adhesive can be applied to a first substrate. The hot melt adhesive can then join the first substrate together with a second substrate as the hot melt adhesive cools back to a solid.
Hot melt adhesives are generally composed of three components: (1) a polymer; (2) a tackifier; and (3) a wax.
The polymer provides strength and adhesive characteristics. The tackifier allows the polymer to be more adhesive by improving wetting during the application. The tackifier is typically necessary in preferred compositions. The wax reduces the overall viscosity of the adhesive, thereby allowing the adhesive to liquify for a hot melt application at lower temperatures. The wax also controls the set time of the system--i.e. the time required to solidify the adhesive once the first and second substrates are compressed together.
Other components such as plasticizers are used in the formulation of hot melt adhesives for various applications. Plasticizers reduce the overall viscosity of the adhesive and promote flexibility and wetting. Typical types of plasticizers used are phthalates, glycolates, polybutenes, and mineral oil. See "Adhesive Bonding", Chap. 8 Hot-melt Adhesives, by Thomas Flanagan, at p. 8-3, published by MacMillan.
Waxes can adversely affect the adhesion characteristics of the polymer. In addition, waxes are not compatible with polymers with high vinyl acetate contents. Thus, plasticizers are used in some applications instead of waxes. Examples of plasticizers are given in U.S. Pat. No. 4,475,026, such as dicyclohexyl phthalate.
Of particular importance is the change in heat, i.e., .DELTA.H, as the hot melt adhesive changes from one phase to another. The lower the .DELTA.H, the less heat is required to convert the solid adhesive to the liquid phase. .DELTA.H is proportional to melt down rate, i.e., the rate of melt down of the adhesive from a solid phase to the liquid phase. Adhesives that melt down to a liquid phase quickly and easily can be used in faster manufacturing applications than adhesives with slower melt down rates. Therefore the lower the .DELTA.H, and the faster the melt down rate, the easier the adhesive is to work with in manufacturing applications. See 1988 Hot Melt, "Thermal Analyses as a Predictor of Performance", by Ernest Eastman, published in 1988 TAPPI Conference Papers. .DELTA.H values can be readily determined using Differential Scanning Calorimeters (Perkin-Elmer).
Another major factor in hot melt adhesive compositions is compatability--i.e. the degree of solubility of one component in another. The greater the compability, the more uniform and homogeneous the adhesive, and the better the bond the adhesive will provide. The closer the solubility parameters of different components are to each other, the greater the compatibility. See "Some Factors Affecting the Solubility of Polymers", by P. A. Small, J. Appl. Chem., 3, Feb. 1953 (which teaches how to predict solubility parameters).
Compatibility is related to cloud point temperature. Cloud point temperature is the temperature at which a component begins to solidify or "cloud up" as it cools from a clear liquid phase to the solid phase. For example, for waxes, the cloud point is usually the melting point of the wax. Generally, the lower the cloud point temperature, the greater the compatibility. See "Adhesives and Coatings Manual" by National Distillers and Chemical Corporation (1983).
As Previously mentioned, set time is the time required to solidify the adhesive once the first and second substrates are compressed together. Fast set times are desirable in most manufacturing applications. Generally, the higher the melting point, the crystallinity, and the viscosity, the faster the set time. Id. Open time is the time during which the adhesive is applied to the first substrate and remains sufficiently molten to effect a bond between the first and second substrate. Long open times are desirable in most manufacturing applications. Generally, the lower the melting point, the crystallinity, and the viscosity, the longer the open time. Id.
Thus, it is desirable to have a highly crystalline adhesive component that has both high-melting-point and low-melting-point characteristics.
The primary object of the present invention is to provide a hot melt adhesive composition containing 1,4-cyclohexane dimethanol dibenzoate as a unique component. The physical properties of 1,4-cyclohexane dimethanol dibenzoate indicate that it would be an improvement over existing hot melt adhesive components such as dicyclohexyl phthalate. More specifically, 1,4-cyclohexane dimethanol dibenzoate provides shorter set times than dicyclohexyl phthalate without adversely affecting open times.
In addition, 1,4-cyclohexane dimethanol dibenzoate has a slightly lower .DELTA.H than dicyclohexyl phthalate. Although other solid benzoates may have a lower .DELTA.H than 1,4-cyclohexane dimethanol dibenzoate, they tend to super cool. During the cool down cycle, these other solid benzoates that super cool remain liquid in the system and act as liquid plasticizers would act. i.e., resulting longer open times and adversely affecting set times. 1,4-cyclohexane dimethanol dibenzoate is highly crystalline and does not super cool.
Furthermore, the 1,4-cyclohexane dimethanol dibenzoate would be very useful as a polyester hot melt modifier. The crystalline nature of the 1,4-cyclohexane dimethanol dibenzoate would contribute significantly to the properties of a polyester hot melt (e.g., faster set time, improved viscosity reduction, etc.). Further, the polyester hot melts containing the 1,4-cyclohexane dimethanol dibenzoate would be useful in applications not now possible without it. Polesters are generally not compatible with waxes and are not easily modified by waxes. U.S. Pat. Nos. 4,172,824 and 4,340,526 teach that certain benzoates are of use in polyester hot melts. However, the cited benzoates are limited in nature due to their tendency to super cool. The 1,4-cyclohexane dimethanol dibenzoate is an improvement over these prior art benzoates.
The use of polyesters comprising 1,4-cyclohexane dimethanol as a hot melt adhesive when blended with certain monomeric ester plasticizers is taught in U.S. Pat. No. 4,340,526. However, this patent does not teach the use of 1,4-cyclohexane dimethanol dibenzoate as a plasticizer in a hot melt adhesive composition.